MDF or Plywood off cuts (Herne Bay)

I am looking for off cuts of MDF or Ply wood to make some chalk boards for a local school garden play area. I have frames so the sizes required are -70 x 50 cm; 2 of 41 x 51 cm; 31 x 31 cm; 49 x 57.5 cm; Obviously anything bigger than these sizes would be great. Happy to collect. Blackboard and gloss paints are also needed to complete the project.Thank you Marion